1. Field of the Invention
This invention relates to a motor driven camera of which the various operations are performed by electric motors as the drive sources.
2. Description of the Related Art
In the art of motor driven cameras, it has been conventional that the electric motor for winding up is used to charge the shutter (for, example,, in U.S. Pat. No. 4,693,578). Since, in this case, the times of driving the film windup and shutter charge transmission systems coincide with each other, for the given motor and electrical power source or battery, determination of the speed reduction gear ratio is made by taking into account the combined load of both mechanisms. That is, on assumption that the actual voltage of the battery lowered to a minimum permissible level and, as the ambient temperature lowered and the number of film frames wound onto the takeup spool was bearing neared the maximum, the load rose to a maximum possible level, the resultant output of the motor could even carry out the winding up of the film and the charging of the shutter at the determined value of the speed reduction gear ratio. As a result, because, in most cases, the determined value of the speed reduction gear ratio was very large, a problem arose that even when the battery was fresh, the framing rate did not speed up. To increase the framing rate, consideration may be given to increase the capacitance of the battery or employ a high performance motor. But, the use of these methods became a cause of increasing the size and cost of the camera itself, and was hardly practiced.
Meanwhile, an improved method has been proposed in U.S. Pat. Nos. 4,272,173, 4,616,913 and 4,647,169 and many others where the speed reduction transmission system is provided with a mechanism for changing over between two gear ratios, thereby it being made possible to selectively set the balance of the actual battery voltage and load at optimum for the characteristic of the motor. With such a technical idea of these patents, however, because the gear ratio changeover mechanism and associated parts therewith were complicated in structure, a problem arose that the cost was increased partly due to the increase of the number of parts. And, the camera system itself was also increased in size.
It is also known to provide for the above-described or gear ratio-changeable type of cameras with another motor for charging in addition to the motor for winding up, as disclosed in U.S. Pat. No. 4,659,201, wherein the starts of energization of these two motors, though made to have a lag in order to prevent the rush currents of both motors from superimposing each other, may be considered to roughly coincide with each other.
When the actual voltage of the electrical power source or battery in the camera falls below the satisfactory operating level, the shutter drive magnet will not normally operate, leaving the film unexposed, or failing to close the once opened shutter with the film in the gate continuing being exposed to the ambient light, or other accidents will happen. To prevent such a faulty operation from occurring, the actual voltage of the battery must be checked before the magnet for driving the shutter is energized, that is, before the shutter runs. If it is determined to be too low to assure performance of the normal operation, the releasing operation is automatically stopped from further going and the photographer is informed by an alarm display of the fact that the battery has exhausted. Such a safe measure has been adopted in many prior known cameras (for example, that of U.S. Pat. No. 4,284,333).
By disusing the conventional rapid-winding lever for advancing the film and charging the shutter in favor of the motorized windup and charge mechanisms or the so-called "winder" built in the camera, the fall of the battery voltage then gives rise to another faulty operation which will take place in a different stage from when the above-described phenomenon appears. That is, if, as the battery voltage falls across the critical level, the motor stops at a time when the film is being wound up, the border of the trailing curtain of the shutter stays within the exposure aperture. Because the shutter of the camera under such a condition exhibits an insufficient ability to shield light, if it is long exposed to intense light, the film will be fogged.
To avoid such an accident, the critical level for the battery voltage may be set a bit too high to increase the safety factor of the normal winding up and charging operations. With this method, however, the release prohibition comes while the voltage is yet high. Therefore, the number of film cartridges shot per battery is decreased. Particularly when in photography with long exposure times of 30 seconds, a few minutes or so, despite that the battery voltage was checked OK before the release, as the voltage has been lowering during the exposure, it is when to actually operate the winder that the normal winding and charging operation fails to perform, or a similar faulty operation to that described above occurs with some possibility.
To eliminate these problems, there has been a previous proposal that, besides the battery checking before the shutter is released, another battery checking is carried out even before the winder is activated, as, for example, in U.S. Pat. No. 4,458,996. But, since, in such a camera, the voltage checking must be twice carried out in every one cycle of releasing and winding operation, for the winder-built-in camera which is adapted to a series of continuous shots, the large drop of the continuous shooting speed (framing rate) constitutes a serious drawback. Why the framing rate is largely dropped is that, in general, when checking the voltage, a real load is put on the battery and an appreciably long time of current flow thereto is waited before to read the value of the voltage. Or otherwise, the ability of the battery could not be tested accurately and reliably. Hence, a surplus time equal to twice the above-identified time becomes necessary to shoot one frame.